I'm sure I set the distribution of asteroids to be Uniform, but take a look at this - this is the setup at the start of the sim. You can see a noticeable gap in the belt around 1 AU that doesn't fill up as the sim goes on... is there something funny going on with the distribution function?

That's probably normal, depending on how many asteroids you specify. It's randomly picking them, rather than creating a continuous distribution, so gaps have a probability of occuring. If you created fewer asteroids, you'd probably have more gaps.

There's a new distribution function in the beta version that lets you specify a gaussian distribution as well, so the middle should be heavily populated, and gaps appear at the edges.